[SPARK-1726] [SPARK-2567] Eliminate zombie stages in UI.

Due to problems with when we update runningStages (in DAGScheduler.scala)
and how we decide to send a SparkListenerStageCompleted message to
SparkListeners, sometimes stages can be shown as "running" in the UI forever
(even after they have failed).  This issue can manifest when stages are
resubmitted with 0 tasks, or when the DAGScheduler catches non-serializable
tasks. The problem also resulted in a (small) memory leak in the DAGScheduler,
where stages can stay in runningStages forever. This commit fixes
that problem and adds a unit test.

Thanks @tsudukim for helping to look into this issue!

cc @markhamstra @rxin
I also moved this event inside the check for tasks.size being > 0 -- because we shouldn't tell the UI/listeners about a stage if it doesn't have any tasks and therefore won't be run.